Acrylonitrile-Butadiene-Styrene-Acrylate copolymers (hereinafter referred to as ABSA copolymers) are styrenic resins comprising a rubbery acrylate-butadiene copolymer phase dispersed throughout a styrene-acrylonitrile resin matrix. Such polymers are known to have excellent impact strengths and improved light stabilities compared to conventional high impact polystyrene (HIPS). They can be produced by adding acrylate monomer and butadiene monomer to a suspension of styrene-acrylonitrile (SAN) polymer beads and polymerizing the resulting admixture.
Difficulties have been encountered, however, in producing ABSA copolymers due to undesired side reactions involving the acrylate monomer and/or the butadiene. The acrylate monomer polymerizes readily with itself to form acrylate homopolymers. Acrylate and butadiene can react with each other to form a cyclic ester often referred to as "adduct." Butadiene can also dimerize to form vinylcyclohexene. The presence of adduct in an ABSA copolymer adversely affects the properties of the copolymer. The polymerization reaction may be interfered with, leaving amounts of unreacted monomer. Polymer integrity and molecular weight may be reduced, impact strengths decreased, sticky polymer beads which tend to form agglomerates produced, and undesirable odor problems encountered.
In attempts to reduce the incidence of adduct formation, reversible chemical inhibitors have been added to the styrene acrylonitrile suspension prior to introduction of the reactive monomers. A deactivating agent for the reversible inhibitor is added to the suspension after the monomer addition has been completed to enable the desired polymerization to take place. For example, it has been attempted to inhibit adduct formation by adding sodium nitrite to the styrene-acrylonitrile suspension prior to introduction of the acrylate monomer and to thereafter deactivate the nitrite inhibitor with ammonium persulfate or sulfamic acid (see Bracke, et al., U.S. Pat. No. 4,247,668). Use of such systems has not proved entirely satisfactory for several reasons. Sophisticated analysis of the suspension is often necessary in order to assure that exactly the right amount of deactivating agent is added to the suspension, thereby precluding operation of the process by unskilled production personnel. Non-uniformities in the reaction rate occur as a result of localization of the effect of the inhibitor or the deactivating agent. Use of such agents can adversely affect the pH required to maintain the suspension and cause the styrene-acrylonitrile beads to precipitate. Production cycle lengths are unduly extended by the time required for analysis and addition of the chemical agents.
There is a need for an improved method for controlling adduct formation in the production of acrylonitrile-butadiene-styrene-acrylate copolymers.